CN110167437B - Pressure sensor - Google Patents

Abstract

An apparatus for pressure sensing, such as a hydrocephalus shunt, has a housing surrounding a chamber and having at least one port in communication with the chamber. The walls of the chamber include flexible portions or sheets of film that flex with a transition in pressure to contact the upstanding structure of the housing. The pressure sensor is contained within a housing isolated from the chamber by a flexible diaphragm and receives fluid pressure from the chamber. This arrangement allows the device to be calibrated by identifying corner features in the pressure data associated with the diaphragm contacting the upstand.

Description

Pressure sensor

Technical Field

The present invention relates to pressure sensors, and more particularly to pressure sensors for long term implantation.

Background

Implanted pressure sensors are known to be used as stand alone devices, or as part of larger devices. A particular problem with implanted sensors is maintaining accurate output. The sensor is typically a relative pressure sensor or an absolute pressure sensor. The relative pressure sensor operates with a reference pressure that can be applied from outside the body. The absolute pressure sensor is self-contained and includes a sealed reference pressure chamber. Typically, the absolute pressure sensor reference chamber is evacuated to minimize the effects of temperature variations, but another known reference pressure may be provided, wherein the temperature will be stable in use, or the temperature sensor is included in a system that can be used to adjust the pressure reading from the sensor.

The relative pressure sensor has a path for externally applying a reference pressure. This is typically a catheter or tubing connected to the sensor and extending from the user's body. This makes the sensor unsuitable for long-term use, or use outside of a clinical setting.

Absolute pressure sensors are known to suffer from drift in sensor output over time. They are susceptible to gradual changes in the response of the mechanical component over time and to changes in the response of the electrical component over time. They are also susceptible to deposit build-up on the components and the housing as well as inside. This drift is not important in the case of short use of the sensor, but may become important in the case of implanted sensors and where long use is required.

Disclosure of Invention

It is an object of at least one embodiment of the present invention to provide a sensor which to some extent overcomes the above-mentioned disadvantages or at least provides a useful choice for the health sector.

In a first aspect, the invention may be considered broadly as an apparatus comprising:

a housing surrounding the chamber and having at least one port in communication with the chamber,

a pressure sensor that receives fluid pressure from the chamber,

the chamber has a compliance that can repeatedly exhibit a significant change in volumetric stiffness at a fixed pressure.

Preferably, the apparatus comprises:

a flexible wall portion forming part of a wall of the chamber,

a sealed cavity separated from the chamber by the flexible wall portion such that increased pressure in the chamber causes increased deflection of the flexible wall portion,

the sealed cavity or housing is shaped such that when the pressure in the chamber transitions to a first pressure, a portion of the wall portion transitions from being in contact with the structure of the cavity or housing to not being in contact with the structure.

Preferably, the flexible wall portion comprises a thin film sheet, and portions of the sheet are capable of continuing to flex at pressures above and below the contact pressure.

Preferably, the flexible wall portion is not in contact with the structure at a pressure lower than the first pressure and is in contact with the structure at a pressure higher than the first pressure.

Preferably, there is no additional detector for determining contact of the flexible wall portion with the housing or cavity.

Preferably, the pressure sensor is located in the housing.

Preferably, the pressure sensor is located within a housing isolated from the chamber by a flexible diaphragm, the housing being filled with an incompressible liquid.

Preferably, at least one port comprises a connection for a flexible tube.

Preferably, the housing comprises an inlet and an outlet, and the inlet and the outlet each comprise a connection for a flexible tube.

Preferably, the apparatus comprises an interface from the pressure sensor to a controller.

Preferably, the device comprises a controller connected to the pressure sensor, an external communication interface connected to the controller, and a power source connected to power the controller.

In another aspect, the invention may be considered broadly to be directed to an assembly comprising an apparatus as described above and having a first flexible tube extending from an inlet and a second flexible tube extending from an outlet.

In another aspect, the invention may be considered broadly directed to a hydrocephalus shunt comprising an assembly as described above and a shunt valve connected to a flexible tube extending from the outlet.

In another aspect, the invention may be considered broadly directed to a system comprising an apparatus as described above, and comprising a processor programmed to process data from a pressure sensor in a calibration method comprising identifying pressure data points from the pressure sensor corresponding to times of compliance change of a chamber.

Preferably, the program identifies the time at which the compliance of the chamber changes by identifying an inflection point (knee) feature in the series of pressure data recorded over the duration of a calibration event.

Preferably, the program identifies the corner feature from a significant change in pressure gradient over time.

Preferably, the program sets a zero offset for use in connection with the pressure sensor based on the pressure output recorded at the identified time.

Preferably, the program sets a zero offset for use in connection with the pressure sensor based on the results of the multiple instances of the calibration method.

Preferably, the program compares the results of multiple instances of the calibration method and discards at least some of the results when calculating a new zero offset for the pressure sensor.

Preferably, the processor may be any one or more of a microprocessor or FPGA device array or Complex Instruction Set Computing (CISC) microprocessor, reduced Instruction Set Computer (RISC) microprocessor or ASIC device or digital signal processor or CPU or desktop or laptop or smartphone or tablet or cloud-based processor or cloud server.

In another aspect, the invention may be considered broadly to be directed to a recalibration method comprising the steps of: a flexible tube closing a port connected to a housing of the pressure sensor moves one of the closures toward the housing.

Preferably, the closure is stably moved toward the housing.

Preferably, the diaphragm within the pressure sensor flexes in response to an increase in pressure within the chamber within the housing to compensate for a loss of volume within the tube due to movement of the closure.

Preferably, the pressure sensor is configured to read the pressure in the chamber.

Preferably, a discontinuity or change in pressure rate is detected and the value output by the pressure sensor is recalibrated based on the detected discontinuity value using the discontinuity.

Preferably, the discontinuity value is used as an offset or bias value to recalibrate the output of the pressure sensor.

It is also to be considered that the invention broadly relates to the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more of said parts, elements or features, and wherein specific integers are mentioned herein which have known equivalents in the art to which the invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

The term "comprising" as used in this specification means "consisting at least in part of. When interpreting each expression of the term "comprising" in this specification, features other than the term or features preceding the term may also be present. Related terms such as "comprising" and "including" will be interpreted in the same manner.

Drawings

The pressure sensor will be described with reference to the accompanying drawings.

FIG. 1 is a cross-sectional view of a pressure sensor housing according to one embodiment of the sensor.

Fig. 2 is a perspective view of the pressure sensor housing of fig. 1.

Fig. 3 is a schematic diagram showing the main parts of the device comprising a housing, a displacer, a sensor, a processor and a communication interface.

Fig. 4a to 4d are diagrams summarizing the operation of the processor in taking sensor readings and calibrating the sensor.

FIG. 5 is a graph plotting transducer output versus time for a test sequence in which pressure is applied with a linear increase.

Fig. 6 is a cross-sectional view of a portion of the transducer of fig. 1, showing in exaggerated form the deflection of the diaphragm at different times in the graph of fig. 5.

Fig. 7 is a diagram showing the position of the sensor device in use.

Fig. 8 is a diagram showing how a system including a sensor device may be physically manipulated when performing calibration.

Fig. 9 is a graph relating pressure in a chamber of a housing to pressure in the housing when performing the calibration of fig. 8.

Fig. 10a to 10f are a series of charts showing the processing of sensor data to identify characteristics.

Fig. 11a shows a graph illustrating corner pressure for a test plan on a sensor.

Figure 11b shows the values of the contact pressure for 10 consecutive presses to illustrate reproducibility.

FIG. 11c illustrates an exemplary experimental setup for performing a test plan on a pressure sensor.

Detailed Description

The present invention relates to pressure sensors, particularly for implantation in the body. The sensor is intended for long term use, for example at least months or years.

The sensor is intended for use in selecting a person who wishes to live outside of a clinical setting and to conduct his daily activities.

For example, the sensor may be used in hydrocephalus patients. This condition involves the possible excessive accumulation of cerebrospinal fluid in the ventricle. This disease affects adults and children and is lifelong. This condition may be controlled by a hydrocephalus shunt. However, shunt failure due to blockage is common. By reliable long-term intracranial pressure monitoring, such conditions, monitoring of the proper operation of any implanted shunt, or both, may be better controlled.

Exemplary devices for this purpose are known in the art. Implantable devices including absolute pressure sensors, processors, and communication interfaces are described, for example, in U.S. patent 6533733 (see, in particular, fig. 3 and 4) and U.S. patent 6248080 (see, in particular, fig. 1 c), the relevant disclosures of which are incorporated herein by reference in their entirety.

The invention proposes adapting the sensor components of these systems to provide a characteristic output curve (plotting the sensor output against the applied pressure) comprising identifiable artefacts that reliably occur under pressure, and a program adapted to be executed by the processor of the device comprising a calibration function identifying the presence of the artefacts.

For example, in a device constructed according to US6248080, the pressure sensor module 20 would include features for generating a characteristic curve including identifiable artifacts, and a program with recalibration functionality would be executed in the microprocessor 120. Alternatively, the program with recalibration functionality may be executed in an external device (e.g., device 500 of FIG. 3) that communicates with the implant device based on sensor data acquired by the implant device. The external device may be any suitable computing device, whether in close proximity to the implanted device or in communication over a network such as the internet.

Referring to fig. 1, one aspect of the present invention includes a sensor component. The sensor component includes a housing 100. The housing 100 includes an interior chamber 102. The housing 100 includes an inlet 104 to the chamber 102 and an outlet 106 from the chamber 102. The outlet 106 is optional. Pressure sensing or calibration does not necessarily require an outlet, but an outlet may be useful when the device is used as part of a hydrocephalus shunt. The inlet and outlet may be substantially identical and the function of either port is not important. The housing may be formed with any suitable connection or connector 108 at the inlet, outlet, or both.

An absolute pressure sensor 110 (e.g., a MEMS sensor) is located in the housing. A pressure sensor 110 is located in the housing in pressure communication with the chamber. The pressure sensor 110 is preferably located in a second space separated from the chamber by the diaphragm 112 in a cavity 114 defined by the housing and the diaphragm 112. The cavity 114 is preferably filled with a substantially incompressible liquid, such as oil. The pressure in the chamber is transferred to the sensor 110 by the oil.

The second cavity 116 is separated from the chamber 102 by a diaphragm 118. The cavity is sealed and contains a gas or vacuum. Preferably, the chamber contains a vacuum. The vacuum is independent of temperature. Alternatively, the chamber may contain a gas at a known pressure. In this case, the volumetric response will also be a function of temperature. The temperature may be measured by a number of MEMS pressure sensors such as LPS22HBTR from ST Microelectronics. The diaphragm 118 may flex in response to the relative pressure between the cavity 116 and the chamber 102, thereby changing the volume of the chamber 102 in response to the fluid pressure in the chamber 102. For example, as the pressure in the chamber 102 increases from the first pressure to the second pressure, the diaphragm flexes to gradually decrease the size of the cavity 116 and increase the size of the chamber 102.

The cavity 116 and the diaphragm 118 are formed such that the volume change between the chamber 102 and the cavity 116 that occurs at a fixed relative pressure with respect to the pressure change has a discernable characteristic. In one embodiment, this is caused by a sudden increase in the effective stiffness of the diaphragm caused by the interaction between the diaphragm and the inner surface of the cavity 116. For example, in the illustrated embodiment, the surface of the cavity includes an upstanding portion 120. As the relative pressure increases, the central portion of the diaphragm eventually rests on the upstand 120. The annular portion of the diaphragm surrounding the upstand 120 continues to flex but the effective stiffness of the diaphragm is increased by supporting the central location.

According to other embodiments, the cavity may comprise a second upright portion that will contact the annular portion of the diaphragm at a second pressure that is higher than the pressure at which the center of the diaphragm rests on upright portion 120. When the membrane rests on the two uprights, the membrane is still able to flex, but is now in the third state of effective stiffness.

According to further embodiments, the contact pressure of the diaphragm and structure may be lower than typically desired so that the diaphragm and structure are normally in contact and out of contact by depressurization during calibration. An example is given later in which a clamping process is performed to change the pressure in the chamber, and subsequently moving the clamping point in one direction will increase the pressure in the chamber, while moving the clamping point in the other direction will decrease the pressure in the chamber. During this exemplary pinching and rolling process, rolling will increase the size of the enclosed space.

According to further embodiments, the contact structure may be disposed in the chamber and contact the chamber side of the diaphragm, rather than the chamber side. In a further embodiment, contact structures may be provided on both sides to contact at different chamber pressures.

This is illustrated in fig. 6, where the diaphragm is shown in a first undeflected position 600 and a second position 602 (dashed line) when the diaphragm has deflected sufficiently to contact the upright 120. The diaphragm is shown in a third condition 604 (dotted line) in which the surrounding portion of the diaphragm has continued to flex under further increased pressure.

The housing may be composed primarily of a hard biocompatible material. For example, the housing may be composed primarily of titanium. The pressure sensor may comprise integrated electronics and communicate through the wall of the housing, so the housing may comprise a radio frequency transparent window, or the housing or a portion of the housing may be formed of a ceramic or polymer such as alumina/zirconia/borosilicate glass.

The second space in which the sensor is located is filled with a pressure transmitting liquid, for example silicone oil. The space is separated from the chamber by a diaphragm that is flexible enough to minimize damping of the transmitted pressure. This may be, for example, a titanium diaphragm having a thickness of less than 25 microns.

The housing surrounding the cavity may also be constructed of a rigid biocompatible material. For example, the housing part may be composed of titanium. The upstand may be integral with the housing or may be a separate component secured in place, for example by adhesive.

The membrane may be a titanium membrane. Preferably, the housing is also titanium so that the cavity can be sealed by welding the diaphragm to the housing (e.g., by laser welding). The chamber may be filled with an inert gas, such as helium. Other inert gases may also be used to fill the cavity.

The housing port may be formed with the housing, for example, formed of titanium. The ports are preferably adapted to connect tubes such as medical grade silicone tubing.

As an implantable device, the sensor component may be configured to a size suitable for a particular purpose. For example, for use in hydrocephalus shunts, the sensor housing may be positioned subcutaneously under the scalp of the patient, on one side or behind the head-such as shown in fig. 7. In this position the device may have a considerable size, for example a total diameter of 10-15mm and a total thickness of 4-7mm.

Where the assembly is included in a system, such as the hydrocephalus shunt of fig. 7, the characteristic volumetric response to pressure changes in chamber 102 may be used to recalibrate pressure sensor 110. In such a system, the sensor assembly 700 is positioned serially in a drain 702 extending from a ventricle 704, preferably above a shunt valve 706, which shunt valve 706 can open or close the drain depending on the pressure conditions in the brain. Accurate pressure readings in the shunt tube near the shunt valve facilitate determining whether the shunt valve is operating properly.

For example, referring to fig. 8, a recalibration method may include clamping or otherwise closing a flexible tube connected to a port of a housing, as at locations 800 and 802. This defines a substantially sealed volume including portions of the connecting tubes 806 and 808 and the chamber 804 containing a largely incompressible fluid (e.g., body fluid). Then, one of the closure members is steadily moved towards the housing, as indicated by closure member 807 and arrow 810, thereby gradually reducing the amount of tube 808 included in the sealed volume. The diaphragm 812 flexes in synchronization with the pressure increase in the chamber 804, compensating for the volume loss in the tube 808 by increasing the volume in the chamber. When this occurs, the sensor 814 reads the pressure in the chamber 804. The steady rate of the clamping movement 802 causes a steady rate of compensation by deflection of the diaphragm 812. The pressure profile introduced into the chamber 804 to induce this deflection depends on the stiffness of the diaphragm. At a fixed pressure, this stiffness increases due to the contact between the diaphragm and the upstand, and the rate of pressure increase in the chamber varies significantly due to the further reduction in the volume of the tube. Illustrated by the graph in fig. 9, which plots pressure in the chamber 804 against volume in the chamber 820. A plot of the pressure measured by sensor 814 versus the time of such a calibration process will show a discernable discontinuity. An exemplary graph is provided in fig. 5. There is a discernible discontinuity in the gradient at location 500.

The output of the sensor may be processed to identify such discontinuities and the sensor system may be recalibrated assuming that the discontinuities occur at a known fixed pressure.

The implant system may include processing and communication facilities. Such a system is illustrated in fig. 3.

The implant system includes a sensor component 11 having one or more internal sensors, including a pressure sensor, wherein the sensing chamber is characterized by a feature 300 (e.g., in accordance with the manner described above) to accommodate increased pressure, thereby exhibiting discernible artifacts. The sensor provides data to the processing device. The processing device may include a controller 13, the controller 13 receiving data from the sensors via the interface 13. The controller 13 may communicate with external devices through a communication interface 15. The system is powered by a power source comprising a power source 16 and a power regulator 21. Such a treatment device is more fully described in US6533733, which is incorporated herein by reference.

In such a system, the sensor will be initially calibrated prior to implantation. Additionally, for the sensor components described above, there may be an initial calibration process to determine the chamber pressure at which chamber compliance exhibits a characteristic change. This pressure is stored as a reference pressure for future calibration events.

The processing of the signals from the sensors may be done by a processing device, by an external device, or by some combination of devices to identify calibration opportunities and to perform a recalibration step. Thus, the methods that will now be described may be implemented in software for an implantable system, in software for an external device in direct or indirect communication with an implantable system, or distributed over some combination of devices.

Fig. 4a to 4d illustrate an exemplary recalibration procedure. Fig. 4a provides the overall process and fig. 4b to 4c provide additional details regarding certain steps in the overall process. The recalibration process is intended to be performed intermittently, wherein the controller is typically used to monitor the activity of the sensor output.

The calibration process begins at step 400, where an initialization is performed. Where the procedure is run in an implantable device, this may be in response to a command received from an external device.

The process proceeds to step 402 to perform a separate calibration event. This is depicted in fig. 4a and is done in parallel with the external influence of the manual boost form. Step 402 generates a pressure dataset that covers a period of a calibration event. At step 404, the process analyzes the pressure dataset to identify a characteristic feature and a sensed pressure (inflection point pressure) at which the feature occurs. This step is described in detail in fig. 4 b. The output of this step is the inflection point pressure at which the feature has been determined to have occurred. Step 406 loops back to perform steps 402 and 404 several ("n") times so that an actual recalibration can occur based on multiple pressurization events to improve accuracy and reliability. Once enough iterations have been made, the process continues at step 408 to discard outliers. This is described in detail in fig. 4 d.

The process then proceeds to step 410 to determine whether to recalibrate. In particular, the process compares the average inflection point pressure at the time of the event occurrence with a reference value based on the current calibration of the sensor to determine the amount of drift since the last recalibration. The amount of drift is compared to a threshold. If the drift is less than the threshold, the process proceeds to step 412, no modification is made to the reference value, and the process ends. If the drift is greater than the threshold, the process proceeds to step 414. The threshold may be set to a clinically significant level. For example, for brain pressure monitoring, the threshold may be set to 2mmHg or some other value.

At step 414, the zero offset of the sensor is reset based on the drift determined at step 410, or based on the difference between the inflection point pressure determined at step 408 and the initial inflection point pressure stored at the initial calibration of the device.

The process preferably also stores the determined drift at step 416 for subsequent analysis of sensor performance over time.

Step 402 is illustrated in more detail with reference to fig. 4 b. At step 430, the implanted device communicates with an external device to set the start of a calibration event. The external device preferably includes a barometric pressure sensor, or data may be obtained from a barometric pressure sensor.

At step 432, the process begins high frequency pressure sensing, with the calibration process desirably occurring over a period of seconds or minutes. This is in contrast to typical monitoring by implanted devices, which may sample pressure over a longer time scale, perhaps taking sensor readings only once an hour. Higher frequency pressure sensing may involve taking readings every second, or taking multiple readings every second, such as 20 readings or more per second. The frequency selected will depend on the desired calibration technique. The frequency must provide enough data for subsequent analysis to reliably determine the inflection point in the pressure versus time curve.

At step 434, the user is prompted to begin a pressurization event. For example, in the case of a pinch/roll calibration procedure, the user may press the closure tube on either side of the pressure sensing component and then begin rolling one finger toward the pressure sensing component.

In steps 436 through 442, the process senses, calculates, and stores pressure data until a threshold pressure is reached. The threshold pressure is substantially higher than the desired inflection point pressure, but lower than the pressure that may damage the sensing component or connection. In the loop, the pressure is read from the pressure sensor at step 436. The air pressure is read from an external air pressure sensor, preferably simultaneously, as in step 438. The difference between the absolute sensed pressure and the air pressure is calculated and stored at step 440, along with the time of reading, or as part of a sequence of known timed readings. At step 442, the process determines whether the threshold has been met and returns to step 436 where the threshold has not been met.

Once the pressure threshold is reached at step 442, the process provides user feedback, preferably in the form of an audible or visual alarm, at step 444. This alerts the user to stop the process of providing increased pressure, as in step 446. The event measurement process then terminates at step 448 by stopping the pressure measurement.

An example process for determining the presence of artifacts in the pressure distribution generated in a calibration event is shown in fig. 4 c. This begins at step 450, for example in the process of FIG. 4b, where the data stored for the pressure calibration event is retrieved. The data set will include a time series of strain gauge pressures.

At step 452, the process may crop out-of-range data. For example, the process may crop one end or the other end, or both, of the data set to include a plurality of entries covering a period of time that the pressure continues to increase from a threshold below the desired inflection pressure to a threshold above the desired inflection pressure. The range is preferably selected to be large enough to explicitly include the inflection point pressure, and sufficient data on either side of the inflection point pressure, to establish the derivative of the fitted curve of data on either side of the desired inflection point pressure. This is shown on the exemplary dataset in fig. 10a, where the data outside the region of interest defined by the lower 900 and upper 902 limits is cropped, leaving a dataset that covers only 17.5 seconds. FIG. 10a also shows a plurality of data points about the best fit line, the data points being displayed as a plurality of dots or points on the graph.

At step 454, the process filters the data set, for example, with a low pass filter, as a first smoothing stage. This is shown in fig. 10b, in relation to the exemplary cropped dataset of fig. 10 a. In fig. 10b, there are a plurality of dots or data points about a line representing unfiltered data points. The line is a low-pass filter applied to the data points, i.e., just like a best fit line. The noise filtered out produces a smoother curve.

At step 456, the process performs a smoothing operation, such as by computing spline smoothing functions to match the data sets. This is shown in fig. 10c, in relation to the filtered dataset of fig. 10 b. As shown in fig. 10c, the data points are shown as dots, i.e. low pass filter outputs. These data points are further smoothed by applying a smoothing spline, as shown by the solid line.

At step 458, the process divides the data curve into portions using "knots" and fits a series of linear piecewise functions over the created knots at step 460. The knot placement step is shown in fig. 10 d. This may use one of many established tools to approximate a given curve in a series of straight lines. The loop of steps 460, 462, 470 and 472 seeks to adjust junction placement to optimize the set of linear fits of the smoothed data using least squares, and to determine a stable measure of inflection point pressure. Ultimately, the optimization should result in the junction being at or near the inflection point pressure point, where the data directly below the inflection point pressure point has a significantly lower gradient than the data directly above the inflection point pressure point. The vertical dashed line in fig. 10d shows an exemplary optimized junction sequence. In this case, 10 knots are placed.

At step 464, the gradient of the sequence of each linear segment is determined. This data is shown in fig. 10e for the exemplary data of fig. 10 d. There are 9 data points involving 9 line segments connected to the 10 junctions of fig. 10 d. In step 466, a change in the gradient data is determined. This first derivative of the gradient data is shown in fig. 10 f. There are 8 data points indicating 8 intermediate junctions where the line segments intersect in fig. 10 d. In this data, the junction where the greatest gradient change occurs is the junction determined to be associated with the pressure inflection point. In the exemplary data, this is the third data point 920, which represents the third intermediate junction 922 in FIG. 10d, occurring at 6.5s.

At step 468, the process records the pressure at the determined junction. In an exemplary dataset, the gauge pressure is 15mmHg, as shown at 924 in fig. 10 d.

Once the loop finds a set of optimized junctions with a stable knee pressure at step 472, the knee pressure is stored as the determined knee pressure for the calibration event at step 474.

Fig. 4d shows in more detail an exemplary process for improving the reliability of recalibration by removing calibration data that may be assumed to be generated by a faulty calibration process. The calibration process is susceptible to imperfect techniques in which the pressure rises in an uneven or non-smooth manner. This may lead to a situation where the process determines that the presence of corner pressure is not a result of proper operation of the sensor component.

In the process of fig. 4d, data from a series of calibration events is processed to determine an average inflection point pressure that excludes outliers. At step 480, the process retrieves corner pressure data for a series of calibration events. The number of events in the series is determined by the loop of step 406 of fig. 4 a.

At step 482, the process selects a subset of values by determining a set of proximate values or selecting a set of values that are proximate to a median of the data set.

At step 484, the process determines an average of the subset of pressure data. The size of the data set may depend on the size of the data set. This may be, for example, the closest 5 values, or the closest half of the values.

At step 486, it is stored and returned as the average inflection point pressure, the overall process of FIG. 4a will be used to determine if a new zero offset is set for the sensor, and if so, the value of the zero offset.

The described process is only one example. Wide variations may be made without departing from the intent of the present invention, and one or more aspects of the process are retained, including: identifying valid calibration data; processing the data to determine characteristic features that have been deliberately induced by the mechanical design of the sensor housing to exist at repeatable pressures; these determined values for sensing are checked by excluding outliers.

Variations in the sensor components are possible, which will lead to corresponding variations in the process. For example, the sensor component may be designed such that the diaphragm exhibits a variety of significant changes in effective stiffness, such as by including more than one interference structure within the cavity, where the structures engage the diaphragm at different chamber pressures. Alternatively, the sensor component may comprise a plurality of chambers separated from the chamber by separate diaphragms and interference structures, each chamber being designed to exhibit a characteristic change in stiffness at a different chamber pressure. In this case, the process will be changed to determine a plurality of inflection pressures, such as an upper inflection pressure and a lower inflection pressure. This may be achieved, for example, by determining a plurality of peaks in the first derivative of the inter-junction gradient.

In another variation, the sensor component may comprise an electronic measurement of the diaphragm contacting the cavity structure, e.g. by capacitance or conductivity, independent of the pressure sensor. This will allow for more direct recalibration of the sensor at any time the chamber pressure exceeds the contact pressure. However, this will preferably operate as an additional recalibration process, while only using sensor pressure for recalibration, as the electronic contact detection itself may be susceptible to drift or other faults. In this case, the recalibration process as defined in fig. 4a to 4d may allow recalibration of the contact pressure of the diaphragm, recalibration of the pressure sensor, or both.

In a further variant, the calibration process comprising closing the inlet and outlet pipes may be performed by means other than pinching the flexible pipe, for example by means of a valve. Furthermore, forcing the pressure increase by decreasing the volume may be accomplished by other means than moving the pinch point on the flexible tube. For example, a plunger or other squeezable chamber or subchamber may be provided.

In another variation, the calibration process may include automation. For example, pinch valves or other valves may be provided and actuated electrically, and pressure increase (volume reduction) may be implemented by a plunger or cam device and actuated electrically.

Referring now to fig. 11a to 11c, tests were performed on pressure sensors similar to those described in this specification. The test includes a series of pressurized tests on a sensor having a similar structure to the sensor described herein. The exemplary test sensor apparatus is similar in structure to the sensor described with reference to fig. 1 and 2, and further includes compliant tubes similar to the connecting tubes 806 and 808 described with reference to fig. 8.

Based on the test results, inflection point pressures (inflection point features), i.e., the presence of discernable discontinuities in the pressure curve, have been understood. In a test plan (protocol), the input pressure of the pressure sensor is generated using a syringe pump. However, this approach is difficult to replicate outside of the laboratory environment. At least one purpose of the test is to try to develop a test plan that can be used while the pressure sensor is in place, i.e. the pressure sensor is in use and implanted in the user.

The second test plan used included pressurizing the pressure sensor by pressing the attached tubing down to the pressure sensor using a human finger.

The pressure generated by finger actuation may be regulated with a constricted flow path to reduce rapid increases in pressure as finger actuation occurs.

For finger-actuated test planning, the characteristic inflection point pressure may be detected by plotting pressure values from the pressure sensors. Gradual pressurization is achieved without intermittent fluctuations. The experimental setup shown in fig. 11c included a pressure sensor. In the exemplary test apparatus, 3 pressure sensors were pressurized and recalibrated simultaneously as part of the test, indicating that the recalibration process could be repeated. The recalibration method used is a finger grip method, as described herein. The test plan also included detecting corner pressure over 10 pressurization cycles. FIG. 11c illustrates an exemplary test apparatus for testing a pressure sensor and illustrates that recalibration may be repeatedly accomplished. As shown in fig. 11c, the test apparatus comprises a pressure source 1110, which may be a syringe or another pressure source. The test equipment also includes a pressure switch 1112 and a pressure sensor 1114. The pressure switch 1112 is in fluid communication with the pressure sensor, and the pressure source 1110 is in fluid communication with the pressure switch 1112.

Fig. 11a shows the inflection point pressure detected from one sensor in 10 consecutive pressurization cycles. As shown in fig. 11a, at about 95 seconds after the start of the pressure calibration test, a red circle 1102 indicates the presence of a discernible inflection point. As shown in fig. 11a, the lighter lines represent piecewise linear fits. The dashed line indicates the optimal junction position, i.e. the pressure junction position. The dark line (made up of multiple points adjacent to each other) is unfiltered pressure data. The best fit line is formed by most of the pressure data.

Fig. 11b shows the inflection point pressure point, i.e. the ground pressure, detected from one sensor in 10 consecutive pressurization cycles. The ground pressure is the pressure value at which the diaphragm contacts the first upright portion. The test plan using the regulator provides a repeatable, improved test plan. The test plan also shows that the sensor device can be recalibrated to an accuracy of approximately plus or minus about 0.1 mmHg. Clinically, this is very useful, since the recalibration method based on finger grip can be reused.

Embodiments of the present invention provide an implantable pressure sensor that can be non-invasively recalibrated in situ. This extends the useful life of the sensor by allowing compensation for long term drift. The compensation is determined based on the performance of the mechanical system that is expected to exhibit minimal drift and variation over time.

While the pressure sensor component and system are described in particular in connection with implantable sensors for use in the medical field, it may also be applicable to other applications requiring a sensor in difficult-to-access locations. Systems that utilize automation of the calibration process would be particularly suitable for applications where the necessary miniaturization may prove to be less challenging.

Claims (19)

1. An apparatus for pressure sensing, comprising:

a housing surrounding a chamber and having at least one port in communication with the chamber,

a pressure sensor that receives fluid pressure from the chamber,

the chamber having a compliance capable of repeatedly exhibiting a significant change in volumetric stiffness at a fixed pressure, wherein the pressure sensor is located within the housing and within a housing sealed from the chamber, the housing being filled with an incompressible liquid, and the pressure within the housing being communicated through the pressure sensor by the incompressible liquid, wherein the apparatus further comprises:

a flexible wall portion forming a portion of a wall of the chamber,

a sealed cavity separated from the chamber by the flexible wall portion such that increased pressure in the chamber results in increased deflection of the flexible wall portion,

the seal cavity is shaped such that when the pressure in the chamber transitions to a first pressure, a portion of the flexible wall portion transitions from being in contact with the structure of the seal cavity to not being in contact with the structure.

2. The apparatus of claim 1, wherein the flexible wall portion forms a portion of a wall of a chamber and the sealed cavity is separated from the chamber by the flexible wall portion such that when a pressure of the chamber increases from the first pressure to a second pressure, a size of the sealed cavity decreases and a size of the chamber increases.

3. The apparatus of claim 1 or 2, wherein the flexible wall portion comprises a thin film sheet, and portions of the film sheet are capable of continuing to flex at pressures above and below the contact pressure.

4. A device according to claim 3, wherein the flexible wall portion is not in contact with the structure at a pressure lower than the first pressure and is in contact with the structure at a pressure higher than the first pressure.

5. The apparatus of claim 1 or 2, wherein there is no additional detector for determining contact of the flexible wall portion with the housing or sealed cavity.

6. The apparatus of claim 1, wherein the incompressible liquid is oil.

7. The apparatus of claim 6, wherein the housing is isolated from the chamber by a flexible diaphragm.

8. The apparatus of claim 1 or 2, wherein at least one port comprises a connection for a flexible tube.

9. The apparatus of claim 8, wherein the housing comprises an inlet and an outlet, and the inlet and the outlet each comprise a connection for a flexible tube.

10. The apparatus of claim 1 or 2, comprising an interface from the pressure sensor to a controller.

11. The apparatus of claim 10, comprising a controller connected to the pressure sensor, an external communication interface connected to the controller, and a power source connected to power the controller.

12. An assembly for pressure sensing comprising the apparatus of claim 8 having a first flexible tube extending from an inlet and a second flexible tube extending from an outlet.

13. A hydrocephalus shunt comprising the assembly of claim 12, and a shunt valve connected with a flexible tube extending from the outlet.

14. A system for pressure sensing comprising the apparatus of any one of claims 1 to 11 and comprising a processor programmed to process data from a pressure sensor in a calibration method comprising identifying pressure data points from the pressure sensor corresponding to the time of compliance change of a chamber.

15. The system of claim 14, wherein the program identifies the time at which the compliance of the chamber changes by identifying a corner feature in the series of pressure data recorded over the duration of the calibration event.

16. The system of claim 15, wherein the program identifies the inflection point feature from a significant change in pressure gradient over time.

17. The system of any of claims 14 to 16, wherein a program sets a zero offset for use in relation to the pressure sensor based on the pressure output recorded at the identified time.

18. The system of claim 17, wherein the program sets a zero offset for use in connection with the pressure sensor based on results of multiple instances of the calibration method.

19. The system of claim 18, wherein the program compares results of multiple instances of the calibration method and discards at least some of the results when calculating a new zero offset for the pressure sensor.

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